Essay: Distance protection relays

The accuracy of distance protection relays from double-circuit lines is highly influenced by the zero sequence mutual impedance and fault resistance. During earth faults with parallel line operation, the impedance computed by the fault locator of the relay will be inaccurate, due to the zero sequence mutual coupling. Also, the power system state and the geometry of the line are important source of error.
An accurate digital distance protection algorithm immune to the reactance effect is approached in the paper. The algorithm uses only the local end information about currents and voltages, and can be implemented in digital distance protections from 110 kV double transmission lines.
In the technique, the deviation angle is calculated and used for changing the position of the fault impedance on the impedance plane. If the residual current of the parallel line can be connected to the measuring input of the relay, the argument can be estimated with only one current distribution factor. Also, the residual current of the parallel line and the residual fault current can be calculated with only two current distribution factors.
Extensive phase to earth shortcircuits with 50 ohm fault resistance were simulated on one circuit of a 110 kV double circuit line. In case that the zero sequence current is calculated, respectively in case that the zero sequence current is injected into relay, the maximum error is up to 1%. The highest error was identified for single phase to earth faults behind the distance protection and also close to relay.
The results of the study indicate that the proposed approach can accelerate the relay operation for clearance of the close-in as well as the high-impedance faults. Moreover, it can provide higher security compared to the conventional nonadaptive decision logic, for the faults occurring at the transmission line boundary. Considering the negligible error generated by the two algorithms, it can be deduced that the distance protection is immune to the reactance effect.
This method is virtually independent of the fault resistance and of the zero sequence mutual impedance. Another advantage of this algorithm is that it does not require informations from the remote end of the circuit, nor source impedance value.
Using this adaptive distance protection scheme in 110 kV double circuit transmission network, the functionality of the distance protection can be improved and the phenomena of overreach and underreach can be avoided. Also, the relay coordination and the selectivity of the entire power system can be enhanced.
ACKNOWLEDGEMENT
This work was co-funded by the European Social Fund through the Operational Programme Human Resources Development 2007-2013 (Contract number POSDRU/159/1.5/S/132395).
REFERENCES
Majid Sanaye-Pasand, Peyman Jafarian (2011). An Adaptive Decision Logic to Enhance Distance Protection of Transmission Lines. Power Delivery, IEEE Transactions